1. Field of the Invention
The field of the invention is a thermodynamic power cycle in which the working fluid (also called motive fluid) is energized by externally applied heat. Within that field, the present invention is a process in which the working fluid in the course of power production reaches a pressure and a temperature above that at which its vapor and liquid have the same density (i.e., a fluid state that is above its critical pressure and temperature, also called supercritical conditions). The present invention is also a process in which the working fluid is other than water or steam. Ammonia is the working fluid of choice for most applications contemplated for the present invention, but other fluid types which, like ammonia, have boiling points below 32xc2x0 F. at a pressure of one atmosphere, absolute, may also be selected for reasons of obtaining greater efficiency, operability, or economics.
2. Description of the Prior Art
Many industrial processes have flowing streams of liquids, solids, or gases that contain heat which must be exhausted to the environment or removed in some way to facilitate proper operation of the process. Typically, the process designer for these industrial processes will use heat exchange devices to capture the heat and recycle it back into the process via other process streams. Often, however, there are not streams suitable to capture and recycle this heat, because they are either already too high in temperature or they contain insufficient mass flow. Any heat which cannot be recycled into the process is typically referred to as waste heat. Most often waste heat is simply discharged to the environment, either directly as an exhaust stream, or indirectly via a cooling medium, such as cooling water.
One method of utilizing waste heat is to raise steam in a boiler to drive a turbine, a known method well recognized by practitioners of the art known as the Rankine cycle. The steam-based Rankine cycle, however, is only economic when it is applied to heat source streams that are relatively high in temperature (generally 600xc2x0 F. or higher) or are large in overall heat content. In other words, high thermal efficiency or significantly large scale is generally needed to make the Rankine cycle economic. A major reason for this is that efficient removal of waste heat from a process stream requires boiling water at multiple pressures/temperatures to capture heat at multiple temperature levels as the heat source stream is cooled. This complexity is costly from standpoints of both equipment cost and operating labor. Overall, the steam-based Rankine cycle is either too expensive or too inefficient or some combination of the two to be applied to streams of small flow rate and/or low-temperature.
Some process developers have substituted other working fluids for steam in the Rankine cycle to obtain greater compatibility with heat source streams of low or moderate temperature. Typically, an organic fluid such as propane is used. Although improved over steam, organic cycles present the same fundamental inadequacies of the Rankine cycle described above.
Accordingly, there is a need for a relatively simple, low-cost, and relatively efficient method of capturing and utilizing waste heat from process streams that are low in temperature or low in overall heat content.
The advantages of using supercritical conditions in a power cycle have been recognized for many years. For example, a 1927 patent (U.S. Pat. No. 1,632,575, Jun. 14, 1927, Abendroth) describes a system for generating power from supercritical steam. Even then the inventor, Abendroth, did not claim supercritical steam power generation as the invention, rather he claimed a variation of it.
Abendroth, ""575, above, highlighted the advantages of supercritical steam generation when he stated, xe2x80x9cThe advantage of this process resides in the fact that a separation of steam and liquid of equal temperatures but of different physical properties cannot take place at any point in the process. In this way the dangers are eliminated which are caused by the well-known ebullition or boiling phenomena.xe2x80x9d In other words, a heated fluid does not boil when it is at a pressure above critical, instead the fluid simply transitions from liquid to vapor as its temperature rises through the critical temperature. Indeed, the properties of the liquid and the vapor are identical at the critical temperature. And, although the dangers of boiling are well-understood and easily controlled in today""s power plants, boiling requires specialized equipment to separate the liquid phase from the vapor phase. Under supercritical conditions, in which no such separation takes place, the equipment is simplified. Moreover, as will be explained later in detail, supercritical operation can have thermal efficiency advantages over boiling operation. Generally, with most working fluids, multiple pressure boiling stages are needed to achieve the same thermal efficiency that supercritical operation can achieve in one stage.
A disadvantage of the supercritical steam cycle is that the heat source must be above 705xc2x0 F., the critical temperature of water. This eliminates many moderate and low temperature heat sources as potential applications for the cycle. A supercritical ammonia cycle, however, is applicable to these heat sources because of the relatively low critical temperature of ammonia, 270xc2x0 F.
The use of ammonia as a working fluid is also known. In U.S. Pat. No. 781,481, Jan. 31, 1905, Windhausen, Jr., a basic form of Rankine cycle is described in which ammonia is the working fluid. The patent covers generally all pure working fluids in which their normal boiling temperature is less than 32xc2x0 F. These xe2x80x9clow boilingxe2x80x9d fluids are, in general, a good match for Rankine cycle applications in which the heat source temperature and/or the condensing temperature is relatively low. Since Windhausen""s patent was first issued in 1905, there have been variations of using ammonia and other low boiling liquids to capture low temperature heat. Many of these were patented during the late 1970""s and early 1980""s at the height of the energy crisis in the U.S. Exemplary of these is an invention to convert natural heat sources (solar, geothermal, etc.) to power (U.S. Pat. No. 4,100,744, Jul. 18, 1978, DeMunari), an invention to produce power from low temperature heat sources in a petroleum refinery (U.S. Pat. No. 4,109,469, Aug. 29, 1978, Carson), an invention to exploit natural temperature differences on the earth such as a mountain top and a desert valley (U.S. Pat. No. 3,953,971, May 4, 1976, Parker), and an invention that is another variation of the use of natural heat sources (U.S. Pat. No. 4,192,145, Mar. 11, 1980, Tanaka).
In 1969, William L. Minto discussed the value of low boiling compounds as working fluids in his patent of a Low Entropy Engine (U.S. Pat. No. 3,479,817, Nov. 25, 1969, Minto). Minto""s engine was in essence a basic Rankine cycle with certain low boiling compounds, mainly halogenated hydrocarbon compounds such as carbon tetrachloride, as the universe of working fluids from which to select. (One example of an acceptable working fluid for Minto""s invention, chlorodifluoromethane, is specifically cited as a potential selection of working fluid for our invention.) Minto""s engine vaporizes the working fluid by ordinary means of boiling at subcritical pressure. In his patent, Minto stated that low boiling compounds could endow a power cycle with certain characteristics when he stated that an xe2x80x9cobject of the present invention is to provide an improved [working fluid] . . . characterized by its efficiency, simplicity, [and] compactness . . . xe2x80x9d. Minto did not recognize, however, that the use of supercritical operating conditions could further increase thermal efficiency of the process and enhance simplicity by eliminating boiling and the attendant boiler equipment.
Only one patent was discovered which mentioned the concept of using ammonia as a working fluid with supercritical operating conditions. In U.S. Pat. No. 3,986,362, Oct. 19, 1976, Baciu, a process is described in which power is generated from a geothermal heat source. The process employs a two step heat transfer arrangement in which hot water from the geothermal heat source warms an intermediate heat storage material, liquid sodium, which in turn heats a pressurized working fluid stream of ammonia to supercritical conditions. The working fluid ammonia is expanded and then reheated by a second two step heat transfer arrangement like that just described. This reheat method increases the efficiency of the cycle but also makes it more complex in that the energy in the heat source stream must be divided in some manner to provide heat to both the primary, or high pressure, turbine""s working fluid and to the reheat, or low pressure, turbine""s working fluid. The cycle includes the generation of low temperature thermal energy as well as electrical energy. The patent claims make no reference to supercritical operation nor to ammonia as the working fluid, although from the detailed description presumably, a supercritical ammonia cycle is a preferred embodiment of the process.
Outside of patent literature, a 1993 paper describes a supercritical ammonia cycle as a simpler and more efficient alternative to both the multi-pressure steam cycle and to the Kalina cycle, which uses variable composition mixtures of ammonia and water in a subcritical cycle [Solomon D. Tetelbaum, xe2x80x9cComparative Characteristics of the High Efficiency Supercritical Bottoming Cycle,xe2x80x9d American Society of Mechanical Engineers, IGTI-Vol. 8, 1993, pp. 445-452]. The main application for the cycle described by Tetelbaum was as a bottoming cycle to capture waste heat from the high temperature ( greater than 900xc2x0 F.) exhaust leaving a combustion turbine. Because the working fluid enters the expansion turbine at a high temperature, near that of the combustion turbine""s exhaust gas, the working fluid leaving the cycle""s expansion turbine is also relatively hot (about 400xc2x0 F.) and thus contains a significant quantity of sensible heat. To make effective use of this sensible heat, a regenerative method is used in which the condensed ammonia from the recirculation pump is preheated by indirect heat exchange with the hot ammonia exhaust. Similar to the reheat method described above, the regenerative method increases efficiency but adds complexity and cost to the cycle.
The present invention is a thermodynamic power cycle system and process to be used for the purpose of extracting a flow of heat from a hot stream of gas, liquid, solid, or mixture of these, hereafter referred to as a heat source stream, and converting a portion of this extracted flow of heat to mechanical power. Heat is extracted from the heat source stream to a working fluid in a heat exchanger in which the two streams flow in opposite (countercurrent) direction. The working fluid flows through the heat exchanger at a pressure above its critical pressure and emerges above its critical temperature. The use of supercritical conditions permits the working fluid to transition from liquid to gas at its critical temperature without boiling. (Or, stated another way, the liquid and gas at the critical temperature have identical properties and are indistinguishable.) This supercritical step simplifies the equipment compared with Rankine cycle by eliminating the boiler section. The boiler section of a Rankine cycle normally contains added equipment of such size and complexity that small heat source streams cannot be practically or economically utilized.
The present invention in its simplest form is comprised of four means to perform four process steps through which the working fluid flows in a closed-loop cycle: (1) means for transferring heat from the heat source stream to the working fluid, (2) means for expanding the working fluid to generate mechanical power and the means for expanding also providing a means for throttling the working fluid to maintain a pressure within the means for transferring heat that is greater than the critical pressure of the working fluid, (3) means for cooling to condense and subcool the working fluid after the means for expanding, and (4) means for returning the working fluid to the means for transferring heat. These are the only four means or steps in the system or process in which energy is added to or removed from the working fluid in the form of heat or work. Since the means used in the system of the present invention so closely parallel the process steps of the present invention, the same will often be treated interchangeably.
Other steps typically used in Rankine cycle, which are often needed to promote good thermal efficiency and which have the drawback of increasing cost and operating complexity, such as multiple stages of boiler pressures, reheat of turbine exhaust, and deaeration, are not necessary in the present invention.
The present invention, by virtue of the properties of the working fluid and the nature of the process flow scheme, is simple, compact, and relatively efficient for heat sources of low to moderate temperature (about 210xc2x0 F. and higher). This combination permits heat to be economically converted to power from heat source streams which are relatively low in heat content, i.e., either low in temperature or mass flow or a combination of the two. One way to express this combination numerically is by using the concept of availability of energy to do work based on the second law of thermodynamics. By this definition, the present invention is economically viable using heat source streams with as low as about 5 million Btu per hour of available heat content (per unit of time). Such streams of low available heat content are not normally economic when used as heat sources with Rankine cycle.
The working fluid of choice is ammonia for those applications in which the heat source stream is in the temperature range of about 400xc2x0 F. to about 700xc2x0 F. This is the range contemplated for many applications of the present invention. However, for heat source temperatures outside this range or for unusual applications within this range, other fluid types may be selected for use in the present invention for reasons of obtaining greater efficiency, operability, or economics.
It is the principal object of this invention to provide a low-cost, simple to operate, relatively efficient, and compact system and process for the conversion of a flow of heat to power.
Detailed objects of the invention are as follows:
It is an object of this invention to provide a system and process which are capable of economically capturing heat from more than one heat source stream within the same facility.
Another object of this invention is to permit, if desired by the user, all of the system and processing equipment (except for the heat transfer means and the heat source stream) to be mounted on one or more portable transportation means.
A related object of this invention is to permit, for a system and process unit of about 4 MW or less in net power output, all of the system and processing equipment (except for the heat transfer means and the heat source stream) to be located on a single portable transportation means.
A still further object of this invention is to permit portable transportation means units to be designed and constructed according to a standardized set of specifications.
It is a further object of this invention to provide a system and process which are sufficiently simple in operation such that it can be started and operated automatically under normal or routine circumstances of operation with the use of conventional computer controls, without benefit of human intervention.
Another object of this invention is to provide a system and process with relatively fast startup capability.
Another object of this invention is to provide a system and process for making power, which, by virtue of their inherent simplicity, may be applied to existing heat source streams within an existing facility without significant change to the operation of the existing facility.
It is an object of this invention to provide a system and process for converting the latent heat of condensing vapors within a heat source stream into power.
A further object of this invention and a more specific application is to utilize this invention to simplify the heat recovery from a hot gas generated by a coal gasifier, and in particular, the type of coal gasifier in which the hot coal gas is quenched in water.
Another object of this invention is that it may be applied as a bottoming cycle to convert waste heat from a topping cycle into power.
It is a further object of this invention that it may be applied to the hot exhaust gas from a combustion turbine system (i.e., application as a bottoming cycle with a combustion turbine as the topping cycle).
A further and more specific object of this invention is that it may be applied to recuperated combustion turbine units, i.e., combustion turbine units in which the hot exhaust has been partially cooled by heat exchange with combustion air.
A still further and more specific object of this invention is that it may be applied to combustion turbines used for the generation of peak loads, i.e., special purpose combustion turbines, also called peaking units, which are employed to rapidly provide electric power to a power transmission grid during intermittent periods in which electric power demand is unusually high.
Still further and more general objects and advantages of the present invention will appear from the detailed description set forth below, it being understood, however, that this more detailed description is given by way of illustration only, and not necessarily by way of limitation since various changes therein may be made by those skilled in the art without departing from the true spirit and scope of the present invention.
Thus, in one aspect of the invention there is provided a thermodynamic power cycle system for extracting a flow of heat from a heat source stream and generating mechanical power from the flow of heat by means of a working fluid flowing within a closed-loop cycle comprising:
means for transferring heat from the heat source stream to the working fluid such that the working fluid warms from a first temperature to a second temperature that is more than 30xc2x0 F. greater than the critical temperature of the working fluid wherein the working fluid has a critical temperature more than 40xc2x0 F. lower than the temperature of the heat source stream and has a normal boiling point less than 32xc2x0 F.;
means for expanding the working fluid and converting work of expansion of the working fluid to mechanical power; said means for expanding and converting work of expansion also throttling the working fluid such that a pressure of the working fluid exceeds the critical pressure of the working fluid by an amount greater than 5% of the critical pressure of the working fluid as the working fluid emerges from the means for transferring heat;
means for cooling to condense and subcool the working fluid after the means for expanding;
means for returning the working fluid to the means for transferring heat;
the means for transferring heat, the means for expanding, the means for cooling, and the means for returning being the only four means in which energy is removed from or transferred into the working fluid in the form of heat or work.
In preferred aspects of the invention:
The heat source stream comprises a gas, liquid, solid or mixture thereof.
There is further provided: an additional means for throttling the working fluid after the means for transferring heat and before the means for expanding; means for controlling the flow rate of the working fluid; means for containing excess of the working fluid in the liquid state after the means for cooling to condense the working fluid; means for redirecting the flow of the working fluid after the working fluid has exited the means for transferring heat to bypass the means for expanding, the means for redirecting the flow containing a means for throttling the working fluid such that a pressure of the working fluid exceeds the critical pressure of the working fluid by an amount greater than 5% of the critical pressure of the working fluid as the working fluid emerges from the means for transferring heat.
A means for increasing the pressure of the heat source stream to restore the pressure lost by the heat source stream as it flows through the means for transferring heat is provided.
Further preferred aspects of the invention include embodiments where: there exists two or more heat source streams, with the thermodynamic power cycle system comprising additional means for transferring heat, each of which means for transferring heat is dedicated to a single heat source stream; wherein the working fluid is divided into separate streams, with each of the separate streams being dedicated to a separate means for transferring heat; and wherein the separate streams of working fluid, after having been heated by transfer of heat from the heat source streams, are combined into a single working fluid stream;
the heat source stream is a gas, and the system further comprises ducting means to transport the gas to the means for transferring heat and wherein the means for increasing the pressure is a fan or compressor;
the working fluid is ammonia, chlorodifluoromethane, sulfur dioxide or bromotrifluoromethane;
the working fluid is ammonia;
the working fluid is chlorodifluoromethane;
the working fluid is sulfur dioxide.
Still further preferred aspects of the invention include embodiments where: all means of the system except for the means for transferring heat are mounted on one or more portable transportation means;
the mechanical power is 4 MW or less and wherein there is only one transportation means;
the working fluid is ammonia for the embodiment wherein there is only one transportation means;
the working fluid is chlorodifluoromethane for the embodiment wherein there is only one transportation means;
And still further aspects of the invention include embodiments where: the heat source stream is a gas which contains a condensable vapor;
the heat source stream is a stream of pressurized hot gas which has been quenched in water;
the stream of pressurized hot gas has been produced by the reaction of coal and oxygen in a coal gasifier;
the working fluid is ammonia for the embodiment wherein the stream of pressurized hot gas has been produced by the reaction of coal and oxygen in a coal gasifier;
the mechanical power is utilized to provide supplemental drive power to an air compressor of an air separation unit, the air separation unit being employed to provide oxygen to the coal gasifier;
the working fluid is ammonia for the embodiment wherein the mechanical power is utilized to provide supplemental drive power to an air compressor of an air separation unit, the air separation unit being employed to provide oxygen to the coal gasifier
And still further preferred aspects of the invention include embodiments where: the heat source stream is generated by a topping cycle;
the topping cycle a comprises a combustion turbine and wherein the heat source stream is exhaust gas from the combustion turbine;
the combustion turbine is a peaking unit;
the working fluid is ammonia for the embodiment wherein the combustion turbine is a peaking unit;
the working fluid is sulfur dioxide for the embodiment wherein the combustion turbine is a peaking unit;
the exhaust gas has been partially cooled by heat exchange with compressed air;
the working fluid is ammonia for the embodiment wherein the exhaust gas has been partially cooled by heat exchange with compressed air.